The objective of this study is to evaluate the safety of HBOC-201 when it is given to sickle cell patients during vaso-occlusive crisis. The mechanism for the severe pain of VOC is hypothesized to be impaired oxygen delivery to the tissues. The current treatment of VOC is analgesia and rest. In this study HBOC-201 or placebo will be given to sickle cell patients who come to UNC with symptoms of VOC. Safety will be monitored via frequent vital signs and blood assessments.